Hemodynamic Performance of a New Punched Stent Strut: A Numerical Study

Endograft-specific hemodynamics after endovascular aneurysm repair: a CFD analysis

Intraluminal prosthetic graft thrombus (IPT) has been described in case of endovascular aortic pathology repair. This study aimed to assess hemodynamic indicators associated with various anatomical morphologies following endovascular aortic repair (EVAR), aiming to offer further references for the choice of clinical therapy. Six model models (normal, iliac compression, aortic compression, aortoiliac compression, iliac distortion, and long-leg stent) were established based on common anatomical morphologies following EVAR. Hemodynamic indicators, such as flow velocity, time-average wall shear stress (TAWSS), oscillatory shear stress index (OSI), and relative residence time (RRT), were captured using computational fluid dynamics (CFD), and the differences between the six models were examined. The peak blood flow velocity at the compressed side iliac artery and the uncompressed side iliac artery corresponding to the aortoiliac artery compression model and the aortic compression model decreased by 30.63% to 48.62%, compared with that in the normal model. Compared with that in the normal model, the peak blood flow velocity at the aorta and the distorted side iliac artery in the iliac distortion model decreased by 7.89% and 41.13%, respectively. The length of the iliac artery stent has little effect on the blood flow velocity. The TAWSS at Iliac grafts showed varying degrees of decline in the other three compression models, particularly in the aortic compression model compared to the normal model. The TAWSS increases at the corner of the artery showing distortion but exhibited a significant decrease toward the distal end of the corner. The areas with higher OSI, and longer RRT were concentrated in the aortoiliac compression model and the iliac distortion model. We found that endograft compression and distortion may be risk factors for IPT. Moreover, the influence of longer stents on the hemodynamics inside stent-grafts is negligible. However, future real-world studies should be conducted to test and verify this speculation.

Numerical analysis of blood flow in a branched modular stent-graft for aneurysms covering all zones of the aortic arch

Due to the anatomical complexity of the aortic arch for the development of stent-grafts for total repair, this region remains without a validated and routinely used endovascular option. In this work, a modular stent-graft for aneurysms that covers all aortic arch zones, proposed by us and previously structurally evaluated, was evaluated from the point of view of haemodynamics using fluid-structural numerical simulations. Blood was assumed to be non-Newtonian shear-thinning using the Carreau model, and the arterial wall was assumed to be anisotropic hyperelastic using the Holzapfel model. Nitinol and expanded polytetrafluoroethylene (PTFE-e) were used as materials for the stents and the graft, respectively. Nitinol was modelled as a superelastic material with shape memory by the Auricchio model, and PTFE-e was modelled as an isotropic linear elastic material. To validate the numerical model, a silicone model representative of the aneurysmal aorta was subjected to tests on an experimental bench representative of the circulatory system. The numerical results showed that the stent-graft restored flow behaviour, making it less oscillatory, but increasing the strain rate, turbulence kinetic energy, and viscosity compared to the pathological case. Taking the mean of the entire cycle, the increase in turbulence kinetic energy was 198.82% in the brachiocephalic trunk, 144.63% in the left common carotid artery and 284.03% in the left subclavian artery after stent-graft implantation. Based on wall shear stress parameters, it was possible to identify that the internal branches of the stent-graft and the stent-graft fixation sites in the artery were the most favourable regions for the deposition and accumulation of thrombus. In these regions, the oscillating shear index reached the maximum value of 0.5 and the time-averaged wall shear stress was close to zero, which led the relative residence time to reach values above 15 Pa-1. The stent-graft was able to preserve flow in the supra-aortic branches.

Modelling of the in-stent thrombus formation by dissipative particle dynamics

BACKGROUND

Stent implantation is a highly efficacious intervention for the treatment of coronary atherosclerosis. Nevertheless, stent thrombosis and other post-operative complications persist, and the underlying mechanism of adverse event remains elusive.

METHODS

In the present study, a dissipative particle dynamics model was formulated to simulate the motion, adhesion, activation, and aggregation of platelets, with the aim of elucidating the mechanisms of in-stent thrombosis.

FINDINGS

The findings suggest that stent thrombosis arises from a complex interplay of multiple factors, including endothelial injury resulting from stent implantation and alterations in the hemodynamic milieu. Furthermore, the results suggest a noteworthy association between in-stent thrombosis and both the length of the endothelial injured site and the degree of stent malposition. Specifically, the incidence of stent thrombosis appears to rise in tandem with the extent of the injured site, while moderate stent malposition is more likely to result in in-stent thrombosis compared to severe or minor malposition.

INTERPRETATION

This study offers novel research avenues for investigating the plasticity mechanism of stent thrombosis, while also facilitating the clinical prediction of stent thrombosis formation and the development of more precise treatment strategies.

Evaluating Short-Term and Long-Term Risks Associated with Renal Artery Stenosis Position and Severity: A Hemodynamic Study

Background: Moderate renal artery stenosis (50-70%) may lead to uncontrolled hypertension and eventually cause irreversible damage to renal function. However, the clinical criteria for interventional treatment are still ambiguous in this condition. This study investigated the impact of the position and degree of renal artery stenosis on hemodynamics near the renal artery to assess the short-term and long-term risks associated with this disease. Methods: Calculation models with different degrees of stenosis (50%, 60%, and 70%) located at different positions in the right renal artery were established based on the computed tomography angiography (CTA) of a personalized case. And computational fluid dynamics (CFD) was used to analyze hemodynamic surroundings near the renal artery. Results: As the degree of stenosis increases and the stenosis position is far away from the aorta, there is a decrease in renal perfusion. An analysis of the wall shear stress (WSS)-related parameters indicated areas near the renal artery (downstream of the stenosis and the entrance of the right renal artery) with potential long-term risks of thrombosis and inflammation. Conclusion: The position and degree of stenosis play a significant role in judging short-term risks associated with renal perfusion. Moreover, clinicians should consider not only short-term risks but also independent long-term risk factors, such as certain regions of 50% stenosis with adequate renal perfusion may necessitate prompt intervention.

A Research on Online Education Behavior and Strategy in University

After the reform and opening up in China, through a series of rapid developments in world, online education has grown both socially and economically. This area has become representative of the fast-growing economy. However, Guangfu culture as a crucial component of Cantonese traditional culture is gradually becoming less influential today. It is the college's responsibility and duty to protect, carry forward, and inherit this traditional culture. Especially during this cyber era, where networks have become a powerful source for communication and study, there are diversified methods of adaptive learning and various learning behaviors. This article aims to analyze the plausibility of adapting an online platform into the teaching arena and the needs of students under this teaching mode. A simulation of applying advanced technology and artificial intelligence into Guangfu culture innovation was also conducted. The contribution shows the users in this platform have a longer study time, compared with non-platform users, and are more interested in traditional culture knowledge than non-users; 21.5% higher in the performance's test.

Hemodynamic Parameters Predict In-stent Thrombosis After Multibranched Endovascular Repair of Complex Abdominal Aortic Aneurysms: A Retrospective Study of Branched Stent-Graft Thrombosis

Background: Branch vessel occlusion is reported in endovascular repair of aortic pathology. This study aimed to evaluate the hemodynamic indicators associated with in-stent thrombosis (IST) of a branched stent-graft (BSG) after endovascular aortic repair (EVAR) of a complex abdominal aortic aneurysm.

Methods: A retrospective evaluation was performed based on the computed tomography (CT) scans and clinical data of three patients who underwent multi-branched endovascular repair. Patient-specific 3-dimensional models were reconstructed, and hemodynamic analysis was performed for IST. Hemodynamics-related parameters including time-averaged wall shear stress (TAWSS), oscillatory shear stress index (OSI), and relative residence time (RRT) were compared among the individual patients.

Results: The flow velocity, TAWSS, OSI, and RRT were radically changed in the area of the IST. In BSGs, IST tended to occur in the regions of hemodynamic alteration near the bends in the device, where a decreased flow velocity (<0.6 m/s) and TAWSS (<0.8 Pa) and an elevated OSI (>0.2) and RRT (>5 s) were consistently observed.

Conclusions: Hemodynamic perturbations in BSGs cause a predisposition to IST, which can be predicted by a series of changes in the flow parameters. Early hemodynamic analysis might be useful for identifying and remediating IST after multibranched endovascular repair.

Retrograde branched extension limb assembling stent of pararenal abdominal aortic aneurysm: A longitudinal hemodynamic analysis for stent graft migration

PURPOSE

Pararenal abdominal aortic aneurysms (PRAAAs) are a life-threatening disease, and hemodynamic analysis may provide greater insight into the effectiveness and long-term outcomes of endovascular aneurysm repair (EVAR). However, the lack of patient-specific boundary conditions on the periphery compromises the accuracy. Windkessel (WK) boundary conditions coupled to hemodynamic follow-up models of a PRAAA patient, aims to provide insights into the link between hemodynamics and poor prognosis.

METHOD

One PRAAA patient underwent EVAR and reintervention after one branch of stent-graft (SG) had migrated. Totally five computational follow-up models were studied. Patient-specific flow data acquired via ultrasound were used to define the boundary conditions in the ascending aorta and the following three branches. Coupled zero-dimensional WK models representing the distal vasculature were used to define the outlet boundary conditions under the abdomen.

RESULTS

Flow divisions of the main SG branches were 40.7% and 24.7%, respectively. Time-averaged wall shear stress and oscillatory shear index (OSI) increased at the junction connected the SG branch and the stent leading to the right common iliac artery (RCIA) where the stent migrated. The OSI and relative residence time (RRT) value in superior mesenteric artery increased notably after the migration, the RRT continuously increased following the reintervention.

CONCLUSION

Unbalanced flow, resulting in locally high-speed flow, high WSS and OSI might significantly affect stent stability. Results suggest that diameters and interconnection design of stents in complex cases should take the flow division into consideration and computational simulations might be considered as a tool for intervention protocol design.

Influence of Hemodialysis Catheter Insertion on Hemodynamics in the Central Veins

Central venous catheter (CVC) related thrombosis is a major cause of CVC dysfunction in patients under hemodialysis. The aim of our study was to investigate the impact of CVC insertion on hemodynamics in the central veins and to examine the changes in hemodynamic environments that may be related to thrombus formation due to the implantation of CVC. Patient-specific models of the central veins with and without CVC were reconstructed based on computed tomography images. Flow patterns in the veins were numerically simulated to obtain hemodynamic parameters such as time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), relative residence time (RRT), and normalized transverse wall shear stress (transWSS) under pulsatile flow. The non-Newtonian effects of blood flow were also analyzed using the Casson model. The insertion of CVC caused significant changes in the hemodynamic environment in the central veins. A greater disturbance and increase of velocity were observed in the central veins after the insertion of CVC. As a result, TAWSS and transWSS were markedly increased, but most parts of OSI and RRT decreased. Newtonian assumption of blood flow would overestimate the increase in TAWSS after CVC insertion. High wall shear stress (WSS) and flow disturbance, especially the multidirectionality of the flow, induced by the CVC may be a key factor in initiating thrombosis after CVC insertion. Accordingly, approaches to decrease the flow disturbance during CVC insertion may help restrain the occurrence of thrombosis. More case studies with pre-operative and postoperative modeling and clinical follow-up need to be performed to verify these findings. Non-Newtonian blood flow assumption is recommended in computational fluid dynamics (CFD) simulations of veins with CVCs.

A numerical method for guiding the design of surgical meshes with suitable mechanical properties for specific abdominal hernias

Abdominal hernia is a common disease, and the most effective treatment of it is using surgical meshes. However, it was found that due to the mismatch of the mechanical properties between the mesh and the tissues around the hernia, there was still a recurrence rate of more than 33% and a high probability of postoperative discomfort. Currently, because of the complex mechanical environment provided by the abdominal tissues and the lack of quantitative research, it is still difficult to select a mesh with suitable mechanical properties for a hernia with specific position and size. Therefore, the purpose of this study is to use numerical models to evaluate the mechanical behavior of both abdominal wall and meshes after repair, and to determine the most suitable mechanical properties of meshes for specific hernias, including elastic modulus and tensile strength. To realize that purpose, defects with different locations and sizes were considered, including defects of 20 mm and 40 mm in diameter on linea alba (LA20 and LA40) and on rectus abdominis (RA20 and RA40), and laparoscopic repair was simulated. The mechanical properties of the most suitable mesh we determined are as follows: suitable modulus for LA20, LA40, RA20 and RA40 are 1 MPa, 3 MPa, 2 MPa and 4 MPa, respectively; suitable tensile strength for LA20, LA40, RA20 and RA40 are 0.35 MPa, 0.89 MPa, 0.77 MPa and 1.43 MPa, respectively. These data could give a standard of mechanical properties which can be referenced in mesh design and evaluation and provide surgeons with treatment advice for specific patients.

HEMODYNAMIC EFFECT OF OBSTRUCTION TO RENAL ARTERIES CAUSED BY STENT GRAFTS IN PATIENTS WITH ABDOMINAL AORTIC ANEURYSMS

To investigate the hemodynamic effects of partial obstruction to the renal orifice caused by inappropriate stent-graft location. Pre-operative and deployment models of a stent graft with various degrees of obstruction to the renal orifice are constructed based on medical images of abdominal aortic aneurysm. Hemodynamics, including flow pattern, time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), as well as relative residence time (RRT) are analyzed using numerical simulations. Flow rate distributions are assessed and verified by in vitro experiments. Results show that partial blockage to the renal branch orifice leads to flow recirculation and vortices with low wall shear stress around the renal ostia, whereas OSI and RRT on the renal arteries increase with the degree of obstruction. While the in vitro experiment indicates a decreasing flow rate to the bilateral renal arteries as renal artery ostia are obstructed. In conclusion, obstruction to the renal arteries induced by an inappropriate stent graft location causes stenosis in the renal artery in the long term. This study reveals a possible pathological mechanism of renal complications due to the implantation of a stent graft.

Numerical and Experimental Investigation of Novel Blended Bifurcated Stent Grafts with Taper to Improve Hemodynamic Performance

The typical helical flow within the human arterial system is widely used when designing cardiovascular devices, as this helical flow can be generated using the “crossed limbs” strategy of the bifurcated stent graft (BSG) and enhanced by the tapered structure of arteries. Here, we propose the use of a deflected blended bifurcated stent graft (BBSG) with various tapers, using conventional blended BSGs with the same degree of taper as a comparison. Hemodynamic performances, including helical strength and wall shear stress- (WSS-) based indicators, were assessed. Displacement forces that may induce stent-graft migration were assessed using numerical simulations and in vitro experiments. The results showed that as the taper increased, the displacement force, helicity strength, and time-averaged wall shear stress (TAWSS) within the iliac grafts increased, whereas the oscillating shear index (OSI) and relative residence time (RRT) gradually decreased for both types of BBSGs. With identical tapers, deflected BBSGs, compared to conventional BBSGs, exhibited a wider helical structure and lower RRT on the iliac graft and lower displacement force; however, there were no differences in hemodynamic indicators. In summary, the presence of tapering facilitated helical flow and produced better hemodynamic performance but posed a higher risk of graft migration. Conventional and deflected BBSGs with taper might be the two optimal configurations for endovascular aneurysm repair, given the helical flow. The deflected BBSG provides a better configuration, compared to the conventional BBSG, when considering the reduction of migration risk.

Numerical and Experimental Investigation of the Hemodynamic Performance of Bifurcated Stent Grafts with Various Torsion Angles

The “crossed limbs” strategy for bifurcated stent grafts (BSGs) is widely employed when abdominal aortic aneurysm (AAA) patients have unfavorable neck or highly splayed iliac arteries. Helical flow is regarded as a typical flow pattern within the human arterial system and is believed to have the positive physiological effects of inhibiting thrombosis formation and atherosclerosis. The “crossed limbs” strategy may induce helical flow and improve the stent graft outcome. To verify the performance of this strategy by considering hemodynamics, we constructed a series of idealized BSGs with various torsion angles and evaluated the hemodynamic performance, including the helical strength, time-averaged wall shear stress (TAWSS), oscillatory shear index, relative resident time (RRT), and displacement force. Our numerical results indicate that an increased torsion angle enhances the helicity strength at the iliac outlets. However, with increasing torsion angle, the TAWSS in the iliac graft decreases and the RRT increases. In addition, our numerical simulations and in vitro experiments reveal that the displacement force increases gradually with increasing torsion angle. In summary, the “crossed limbs” strategy may have benefits for AAA treatment in terms of helical flow, but because of the unfavorable hemodynamic performance verified by analyzing the hemodynamic indicators, the risk of stent graft migration increases with increasing torsion angle. Therefore, the “crossed limbs” strategy should be carefully employed in surgical AAA treatment.

Hemodynamic performance within crossed stent grafts: computational and experimental study on the effect of cross position and angle

Background and aims

The crossed limbs stent graft technique is regularly employed to treat abdominal aortic aneurysm patients with unfavorable aneurysm necks or widely splayed common iliac arteries. This article numerically evaluates the hemodynamic performance of the crossed limbs strategy by analyzing numerical simulations and conducting experiments using two series of idealized bifurcated stent grafts with different cross angles and cross positions.

Results

Results demonstrated that the absolute helicity at outlets decreased with increased cross angles and increased with decreased cross positions. The time-averaged wall shear stress remained approximately unchanged, whereas the oscillating shear index and relative resident time decreased slightly when the cross angle increased and cross position decreased in iliac grafts. Additionally, both numerical and in vitro experimental results indicate the displacement force acting on the stent graft gradually increased as cross angles increased and cross positions decreased. Results further indicated that strip areas with a high oscillating shear index and high relative resident time, which may be vulnerable to thrombosis formation, exist along the outer surface of the iliac artery grafts.

Conclusion

Given this information, the optimal crossed limbs configuration may contain a small cross angle and low cross position; however, low cross positions may increase the risk of migration.

Artificial Organs 2016: A Year in Review

In this Editor's Review, articles published in 2016 are organized by category and briefly summarized. We aim to provide a brief reflection of the currently available worldwide knowledge that is intended to advance and better human life while providing insight for continued application of technologies and methods of organ Replacement, Recovery, and Regeneration. As the official journal of The International Federation for Artificial Organs, The International Faculty for Artificial Organs, the International Society for Mechanical Circulatory Support, the International Society for Pediatric Mechanical Cardiopulmonary Support, and the Vienna International Workshop on Functional Electrical Stimulation, Artificial Organs continues in the original mission of its founders "to foster communications in the field of artificial organs on an international level." Artificial Organs continues to publish developments and clinical applications of artificial organ technologies in this broad and expanding field of organ Replacement, Recovery, and Regeneration from all over the world. We were pleased to publish our second Virtual Issue in April 2016 on "Tissue Engineering in Bone" by Professor Tsuyoshi Takato. Our first was published in 2011 titled "Intra-Aortic Balloon Pumping" by Dr. Ashraf Khir. Other peer-reviewed Special Issues this year included contributions from the 11th International Conference on Pediatric Mechanical Circulatory Support Systems and Pediatric Cardiopulmonary Perfusion edited by Dr. Akif Ündar and selections from the 23rd Congress of the International Society for Rotary Blood Pumps edited by Dr. Bojan Biocina. We take this time also to express our gratitude to our authors for offering their work to this journal. We offer our very special thanks to our reviewers who give so generously of time and expertise to review, critique, and especially provide meaningful suggestions to the author's work whether eventually accepted or rejected. Without these excellent and dedicated reviewers the quality expected from such a journal could not be possible. We also express our special thanks to our Publisher, John Wiley & Sons for their expert attention and support in the production and marketing of Artificial Organs. We look forward to reporting further advances in the coming years.

Improvement of hemodynamic performance using novel helical flow vena cava filter design

We propose a vena cava filter in which helical flow is created in the filter’s working zone to minimize filter blockage by trapped clots and facilitate the lysis of trapped clots. To validate this new design, we compared five helical flow inducers with different thread pitches in terms of blood flow patterns in the filter. The vena cava was reconstructed based on computed tomography images. Both the numerical simulation and in vitro experiment revealed that the helical flow inducer can effectively create a helical flow in the vessel, thereby subduing the filter structure’s adverse disruption to blood flow, and increasing flow-induced shear stress in the filter center. In addition, the smaller thread pitch helical flow inducer reduced the oscillating shear index and relative residence time on the vessel wall. Moreover, we observed that the helical flow inducer in the vena cava could induce flow rotation both in clockwise and counterclockwise directions. In conclusion, the new design of the filter with the smaller thread pitch inducer is advantageous over the traditional filter in terms of improving local hemodynamics, which may reduce thrombosis build-up after deployment.